Ondansetron is a kind of drug which is used for the treatment of nausea and vomiting induced by cytotoxic chemotherapy and radiotherapy. In this paper, a method to determine Ondansetron in injection by high performance capillary zone electrophoresis is discussed. The injection was directly introduced into the capillary employing 40 mmol/L phosphate at pH 4.03 as buffer. The capillary used was 75 microns in i.d. and 47 cm in length. UV detection was set at 254 nm. The calibration curve showed good linearity. The recovery of Ondansetron ranged from 95%-98% with a coefficient of variation of 1.05%(n = 4). This method is simple, rapid and reliable.